1. Field of the Invention
The present invention relates to a mask for forming a selective grating and selective area growth and a method for fabricating an electro-absorption modulated laser device by utilizing the mask.
2. Description of the Related Art
In general, an electro-absorption modulated laser (referred to as EML hereinafter) device used for optical communications utilizes Franz-Keldysh effect, and has a structure in which a laser diode and a modulator are integrated on the same substrate. In addition, the EML device structurally has a diffraction grating formed in a laser diode region to maintain a single mode even during a high speed modulation.
FIG. 1 is a partially cut away perspective view illustrating an example of an EML device. As shown in FIG. 1, an EML has a distributed feedback laser diode I and an electro-absorption modulator II which are integrated on the same substrate 10 with an electrically separating region disposed therebetween. A diffraction grating 10g is formed in the laser diode region I of the substrate 10, and not in the modulator region II. On the substrate 10 in which the selective diffraction grating 10g is thus formed, basic epitaxial layers 11 including an active layer are formed in an embedded configuration, current blocking layers 12a and 12b are formed at sides of the basic epitaxial layers 11. In addition, a clad layer 13 and a cap layer 14 are formed on the basic epitaxial layers 11 and the current blocking layers 12a and 12b, electrodes 15 are formed on the cap layer 14. In such EML device, the high speed modulation can be performed by causing light emitted from the distributed feedback laser diode I to be absorbed in the modulator II or pass through the modulator II according to whether a voltage is applied to the modulator or not while the distributed feedback laser diode I is continuously operated.
A method for fabricating such EML device will be described as follows. First, the selective grating 10g is formed on the region I of the substrate 10 on which the distributed feedback laser is formed. To this end, after a silicon dioxide (SiO.sub.2) film is deposited on the substrate 10, a predetermined pattern is formed by using conventional lithographic process employing a mask in which a pattern for the selective grating is formed. Then, the mask of the silicon dioxide film for forming the selective grating is removed.
Next, after a silicon dioxide is deposited again, a selective area growth mask is formed by a conventional photolithographic method. The epitaxial layers 11 which comprises an active layer of a multiple quantum well structure and lower and upper clad layers formed at the lower and upper sides of the active layer for lasing are grown by using the selective area growth mask. Then, after mesa etching is performed to form a strong index guiding structure, the current blocking layers 12a and 12b for current confinement are formed at the etched portion. Next, after the upper clad layer 13 is regrown, and the cap layer 14 is formed thereon, the electrodes 15 are formed to complete the fabrication of the EML device.
In the EML device fabricated as above, the grating plays an important role in causing the EML device to operate in a single mode during the high speed operation. That is, it is important that the pitch of the grating is precisely formed for an emitted laser beam to form a single mode beam. FIGS. 2 and 3 show patterns of conventional selective grating masks for forming a grating of the above EML device. FIG. 2 shows a pattern of a stripe type mask, and FIG. 3 shows a pattern of an island type mask.
First, a method for fabricating a grating with a stripe type mask will be described as follows.
After a SiO.sub.2 thin film is deposited on a InP substrate 10, a stripe type mask 100 is formed by patterning the film as shown in FIG. 4.
Next, as shown in FIG. 4B, photoresist is deposited on the substrate 10 provided with the mask 100, and exposure utilizing a holographic effect is performed. By doing this, incident light rays on the photoresist surface form a grating pattern in which low and high illumination regions are repeatedly formed at constant distances by interference between the incident light rays from two directions. Accordingly, the exposed photoresist film has different exposure depths in a grating pattern as shown in FIG. 4B.
Next, when the exposed photoresist film has been developed, deeply exposed portions are removed, and as shown in FIG. 4C, the substrate appears in a grating pattern.
Since the pattern of the photoresist film developed as above has rough figures and is hardly used as it is, the edge portions of the pattern are slight burned out with an O.sub.2 asher, and the figures of the pattern are made to be precise as shown in FIG. 4D.
Next, when the substrate 10 is etched by wet or dry etching, a grating is formed, and the photoresist is removed as shown in FIG. 4E.
However, if the grating is formed by using such a stripe type mask, a stepped portion is formed between the region in which the grating is formed and the region in which the grating is not formed. FIG. 5 shows the whole appearance of the grating formed by the above process, and FIG. 6 shows a section view taken along line A-A' of FIG. 5. As shown in FIG. 6, a height difference H occurs between the region 10 in which the grating is formed and the region 20 in which the grating is not formed. This occurs because etchant on the silicon dioxide film used as a mask during etching is concentrated in the vicinity of the boundary of two regions while the etchant diffuses toward the region 10 in which the grating is formed. Therefore, etching rate at the boundary is highly increased, and accordingly, etching occurs deeply to form the height difference H. In particular, when the width of a patterned silicon dioxide film is wider, such height difference H becomes larger since more etchant diffuses toward the boundary. In case that such height difference H occurs, when a selective area growth mask pattern 30 is subsequently formed on selected areas of the grating region 10 to grow a laser oscillating layer as shown in FIG. 7, there is a problem in which the selective area growth mask pattern 30 may have discontinuities at the boundary as shown in FIG. 8.
In order to improve such height differences, a method for forming a grating by using the island type mask pattern was proposed. In case that the stripe type mask is used, the width of the stripe is reduced to reduce the quantity of the etchant diffusing toward the boundary. However, when the stripe type pattern is used as a mask, the width of the stripe cannot be reduced since the width of the stripe is the length of the modulator. When an island type mask in which an island pattern is formed to have a length corresponding to the length of the modulator and a gap corresponding to the length of the laser diode is used, the problem of the height differences can be solved. However, even in case that such island type mask is used, there is still a problem. There is almost no problem in forming a grating in a selected area, but in case that a selective area growth mask pattern 30i for epitaxial growth for laser oscillation is formed in the selected area on the grating as shown in FIG. 9, the surface of the mask pattern is not clearly defined.
FIGS. 10A and 10B are photographs by a scanning electron microscope (SEM) illustrating problems occurring in fabricating an EML device by a grating forming method using a conventional stripe or island type mask. That is, FIGS. 10A and 10B shows problems arising when a selective area growth mask for selectively growing an epitaxial layer for laser oscillation, in which a grating is formed by using a stripe or island type mask is made of SiO.sub.2. FIG. 10A shows a case in which a grating is formed by using a stripe type mask, and it can be found that the height differences seriously occur at the boundary between the region in which the grating is formed and the region in which the grating is not formed, and a SiO.sub.2 mask pattern for selective area growth is bent at the boundary and the SiO.sub.2 mask pattern on the grating is not precisely formed. On the other hand, FIG. 10B shows a case in which a grating is formed by using an island type mask, and it can be found that though the problem concerning the height difference at the boundary is improved, a SiO.sub.2 mask pattern for selective area growth is still not smartly formed on the grating.